Might be you have manually truncated the log in primary database
using dbcc settrunc(ltm,ignore)

To correct this problem
go to RSSD server , use RSSD database & issue following command

rs_zeroltm ,
go

go to primary server . use primary db and issue following

sp_start_rep_agent PRIMARY_DB
go

Note :

The rs_zerotlm stored procedure will never cause loss of data. It instructs Replication Agent to go back to the database and use the secondary truncation point as the starting point for scanning the transaction log instead of the last page/row that Replication Server received.